# Filter MoveIt cartesian path plans with jump_threshold

MoveIt is a useful tool for robot motion planning, but it often lacks documentation for key functions and features. One particularly opaque function is compute_cartesian_path, which takes as input waypoints of end effector poses, and outputs a joint trajectory that visits each pose. Although this function is in the tutorials, the tutorials don’t cover the critical role of the jump_threshold parameter when running the function on a real robot.

On real robots, executing the trajectory produced by compute_cartesian_path(..., jump_threshold=0) can sometimes cause the robot to jerk unpredictably away from the desired path. This undesirable motion seems to occur when the arm approaches singularities and can’t continue following the waypoints without reorienting the arm.1

The solution to this problem is to set the jump_threshold parameter, which limits the maximum distance or “jump” in joint positions between two consecutive trajectory points. When this parameter is set, the planner returns a partial solution if consecutive joint positions exceed the threshold, so the robot will no longer move jerkily.

But what should the value of jump_threshold be? Looking at the code, the trajectory points must satisfy the following condition:

distance between consecutive joint positions <= jump_threshold * mean joint position distance


So the distance must be less than or equal to the threshold times the mean distance.2 In practice, I played with the threshold3 until the robot moved without jerking or stopping prematurely.

Tuning the jump_threshold was critical for getting cartesian path planning working on our robot arm. Now, we plan the whole trajectory up front and abort if only a partial trajectory is found. Hope this helps!

Footnotes

1. In most cases, the cartesian planner will recognize that there is no feasible plan and return a partial trajectory. But sometimes it seems to attempt to reorient the arm quickly, which causes the jerky deviation from the desired path.

2. There is code for using an absolute distance condition instead of the mean. Also see this Github issue for relevant discussion.

3. I use jump_threshold=5.0 but your mileage may vary.

Tags: how-to, robotics